Preheat your oven to 250°F (120°C). Line a large baking sheet with parchment paper for easier cleanup.
In a very large bowl, combine the Rice Chex, Corn Chex, Wheat Chex, pretzels, and mixed nuts. Gently toss to distribute evenly.
In a separate small bowl, whisk together the melted butter, Worcestershire sauce, seasoned salt, garlic powder, onion powder, and black pepper until well combined.
Pour the butter mixture over the cereal and nut mixture. Stir gently until all the pieces are evenly coated. This might take a minute or two to ensure every nook and cranny is seasoned.
Spread the coated mixture in a single layer onto the prepared baking sheet.
Bake for 45 minutes to 1 hour, stirring every 15 minutes. The goal is to toast the mixture until it’s golden brown and crispy. Be careful not to burn it.
Remove the baking sheet from the oven and let the Chex Mix cool completely on the baking sheet. It will become crispier as it cools.